(1) Field of the invention
The present invention relates to a nickel-metal hydride storage cell, and more particularly to a nickel-metal hydride storage cell which employs a positive electrode containing a nickel hydroxide active material whose particles are coated with a cobalt compound and a negative electrode containing a hydrogen-absorbing alloy, and a manufacturing method of the cell.
(2) Description of the Related Art
A nickel positive electrode for use in a nickel-metal hydride storage cell can be either a sintered type or a paste type (non-sintered type). The sintered type is produced by filling a nickel powder-sintered substrate with an active material, whereas the paste type is produced by filling a highly porous nickel substrate such as a nickel fiber-sintered porous member or a foam nickel porous member with an active material paste.
The sintered type has drawbacks that the filing operation of an active material is complicated and that it is hard to enhance the energy density of the electrode because there are limits to an increase in the porosity of the substrate. In contrast, the paste type is easy to handle, which allows a high-density filling. For this reason, the paste type nickel electrode has become the mainstream, as the demand for higher energy density and lower cost of cells grows.
In spite of these advantages, however, the paste type nickel electrode has the following disadvantages: the electric contact between the active material and the substrate is insufficient because the pores in the substrate have large diameters. Consequently, the electrode has poor efficiency in electricity collection, and the generating ability of the high-density active material cannot be fully brought out.
In order to overcome these drawbacks of the paste type nickel positive electrode, various techniques have been suggested as follows: (1) Japanese Laid-open Patent Application No. 62-222566 discloses a technique of forming cobalt hydroxide layers over the surfaces of solid solution active material powder particles containing nickel hydroxide and either cadmium hydroxide or cobalt hydroxide. (2) Japanese Laid-open Patent Application No. 3-62457 discloses a technique of forming a solid solution of nickel hydroxide and cobalt hydroxide onto the surfaces of nickel hydroxide particles. (3) Japanese Laid-open Patent Application No. 5-151962 discloses a technique of forming hydrophilic organic layers onto the cobalt compound-coated layers which are formed onto the nickel hydroxide power particles, as an improved technique of the above-mentioned Japanese Laid-open Patent Application No. 62-222566.
These techniques have successfully improved the electric conductivity among active material particles and increased the active material utilization rate of the nickel positive electrode, thereby expanding the capacity of the nickel positive electrode. However, the expansion of the nickel positive electrode capacity does not directly lead to the improvement of the performance of an alkali-nickel storage cell. The reason for this is as follows.
As the active material utilization rate grows, the actual capacity of the positive electrode expands. However, when this positive electrode is used with a conventional negative electrode, the excess capacity (charge reserve) of the negative electrode reduces in proportion to the expansion of the actual capacity of the positive electrode. Consequently, more hydrogen dissociates from the negative electrode during a charging operation, which causes the internal pressure of the cell to be raised. Furthermore, the negative electrode performance deteriorates with the progress of the charge/discharge cycle, and the domination of the positive electrode is easily collapsed. The increase in dissociated hydrogen and the collapse of the positive electrode control cause the safety valve to operate to release the electrolyte to the outside the storage cell, thereby deteriorating the cycle life of the cell.
Therefore, in order to lead the expansion of the capacity of a nickel positive electrode to the improvement of the performance of a alkaline nickel storage cell, it is necessary to employ a negative electrode which is suitable for the performance of the nickel positive electrode, and to balance the capacities of these electrodes.
The object of the present invention is to provide a nickel-metal hydride storage cell which has a large actual capacity and excellent cycle characteristics, and restricts a rise in the cell internal pressure during a charging operation, by using a nickel positive electrode having a high active material utilization rate and a hydrogen-absorbing alloy electrode which has an excellent low-temperature discharge characteristic, and by balancing these electrodes.
In order to achieve the object, the nickel-metal hydride storage cell of the present invention comprises a non-sintered nickel positive electrode which is filled with a cobalt-coated nickel active material including mother particles exclusively or mainly composed of nickel hydroxide, and cobalt compound layers partly or entirely coating the surfaces of the mother particles; a metal hydride negative electrode which is filled with a hydrogen-absorbing alloy which absorbs and desorbs hydrogen; and an electrolyte which includes an alkali aqueous solution. In the nickel-metal hydride storage cell, a positive electrode non-reactive capacity rate is 16% or lower and a negative electrode charge depth is 80% or lower after an initial charge/discharge operation.
The positive electrode non-reactive capacity rate is determined by a following equation 1:
positive electrode non-reactive capacity rate %=(positive electrode theoretical capacityxe2x88x92actual cell capacity)/positive electrode theoretical capacityxc3x97100xe2x80x83xe2x80x83Eq. 1.
The negative electrode charge depth is determined by a following equation 2:
negative electrode charge depth %=(negative electrode remaining capacity+actual cell capacity)/negative electrode whole capacityxc3x97100xe2x80x83xe2x80x83Eq. 2.
The nickel-metal hydride storage cell of the present invention can be manufactured by a method comprising the following four steps: a first step of producing a cobalt-coated nickel active material by dispersing mother particles exclusively or mainly consisting of nickel hydroxide into a cobalt compound-contained solution, and by precipitating a cobalt compound by adding an alkali solution to the cobalt compound-contained solution with a pH value being adjusted; a second step of applying a heat treatment to the cobalt-coated nickel active material by adding an alkali metal solution to the cobalt-coated nickel active material and by heating the cobalt-coated nickel active material in a presence of oxygen; a third step of producing a non-sintered nickel positive electrode whose non-reactive capacity rate expressed by the Equation 1 is 16% or lower, by using the cobalt-coated nickel active material containing the cobalt compound which has been heat-treated in the second step; a fourth step of assembling a nickel-metal hydride storage cell whose negative electrode charge depth expressed by Equation 2 after an initial charge/discharge operation is restricted to 80% or lower, by using the non-sintered nickel electrode and a metal hydride negative electrode which is filled with a hydrogen-absorbing alloy.
The present invention will be detailed hereinafter, based on the above-explained method of manufacturing a nickel-metal hydride storage cell.
In the first step, nickel mother particles are dispersed into a solution which contains a cobalt compound, and the pH of this solution is adjusted to a predetermined value. As a result, the cobalt compound is precipitated in the manner that it coats the surfaces of the nickel mother particles.
In the second step, the nickel mother particles which have been coated with the cobalt compound precipitate is soaked in an alkali metal solution, and heat-treated in the presence of oxygen. As a result, the oxidation number of the cobalt contained in the cobalt coating layer becomes greater, and at the same time, its crystalline structure is disordered. Thus, a cobalt-coated nickel active material has excellent electric conductivity and electrolysis perviousness.
In the third step, the cobalt-coated nickel active material is used to fill a porous nickel substrate, thereby producing a non-sintered nickel positive electrode whose non-reactive capacity rate which is represented by the Equation 1 is 16% or lower. Since the cobalt-coated nickel active material has excellent electric conductivity and electrolyte perviousness and high electrochemical activity, the use of this active material makes it possible to obtain a positive electrode whose non-reactive capacity rate is 16% or lower
The value of the actual cell capacity which appears in the Equation 1 was found in a positive electrode-controlled cell system, and the value of the positive electrode theoretical capacity was obtained from the Equation 3 under the conditions that the valence of nickel hydroxide changes between divalence and trivalence in the charge/discharge reaction with the capacity per unit weight of 289 mAh/g.
Positive electrode theoretical capacity=289 mAh/gxc3x97the amount (g) of nickel hydroxide in the positive electrodexe2x80x83xe2x80x83Eq. 3
In the fourth step, a nickel-metal hydride storage cell whose negative electrode charge depth represented by the Equation 2 is restricted to 80% or below is produced by using the non-sintered nickel positive electrode, together with a hydrogen-absorbing alloy electrode (metal hydride negative electrode) and an alkali electrolyte. The storage cell thus produced has the following features.
When the negative electrode charge depth is 80% or below, the negative electrode excess capacity is sufficient, so that less hydrogen gas dissociates from the negative electrode during a charging operation. Consequently, the hydrogen gas pressure is not strong enough to cause the safety valve to operate, and as a result, there is no deterioration of the cell performance or the cycle characteristic due to a reduction in the electrolyte.
Thus, the present invention combines a high-performing nickel positive electrode whose non-reactive capacity rate is 16% or lower with a hydrogen-absorbing alloy electrode which can restrict its charge depth to 80% or below. The combination makes it possible to expand the actual cell capacity and to restrict the occurrence of hydrogen gas at the negative electrode, thereby realizing a cell with a high capacity and a long cycle life.
Such a construction of the present invention will be further detailed as follows with reference to FIG. 1.
FIG. 1 shows the capacity construction of a storage cell. In FIG. 1 the positive electrode theoretical capacity is represented as the sum of the nickel hydroxide non-charge/discharge capacity (a), the actual cell capacity (b), and the nickel hydroxide non-discharge capacity (c). The negative electrode whole capacity is represented as the sum of the negative electrode excess capacity (x), the actual cell capacity (b), and the negative electrode remaining capacity (y). The negative electrode remaining capacity CV) consists of y1 and y2. The y1 corresponds to the nickel hydroxide non-discharge capacity (c) and the cobalt compound non-discharge capacity (d), whereas the y2 indicates the capacity due to the oxidation reaction other than positive electrode one.
The nickel hydroxide non-charge/discharge capacity (a) indicates volume which is used neither for charge nor discharge. The nickel hydroxide non-discharge capacity (c) and the cobalt compound non-discharge capacity (d) indicate volume which is used for charge but not for discharge. However, the cobalt compound non-discharge capacity (d) indicates the charge capacity (oxidation capacity) of the cobalt compound that is added for the purpose of improving the active material utilization rate and does not contribute to the discharge, so that it is excluded from the positive electrode theoretical capacity.
Since the cobalt contained in the cobalt-coated nickel active material of the present invention has become higher-order through an alkali heat treatment, only small power is demanded during a charging operation. Furthermore, the cobalt coating layer containing the high-order cobalt compound has high conductivity and excellent wettability against an electrolyte due to its disordered crystalline structure. Consequently, the cobalt-coated nickel active material has a high utilization rate, and the nickel positive electrode which is filled with such as cobalt-coated nickel active material has a low non-reactive capacity rate. Thus, in FIG. 1 the nickel hydroxide non-charge/discharge capacity (a), the nickel hydroxide non-discharge capacity (c), and the cobalt compound non-discharge capacity (d) can be reduced whereas the actual cell capacity (b) can be expanded in the present invention. To be more specific, the positive electrode non-reactive capacity rate which is defined in the Equation 1 can be set at 16% or lower, and as a result, the nickel positive electrode has an extremely high electrode capacity.
We inventors of the present invention have confirmed that in the conventional non-sintered nickel positive electrode which is filled with an active material containing a mere mixture of nickel hydroxide and cobalt compound powder, the non-reactive capacity rate is 19.0% or higher.
If a cell is manufactured by using the high-performing nickel positive electrode whose non-reactive capacity rate is 16% or lower and the conventional negative electrode, then the nickel hydroxide non-charge/discharge capacity (a), the nickel hydroxide non-discharge capacity (c), and the cobalt compound non-discharge capacity (d) are reduced and the actual cell capacity (b) is expanded, and accordingly, the negative electrode excess capacity (x) and the negative electrode remaining capacity (y) are reduced. The reduction in the negative electrode excess capacity (x) and the negative electrode remaining capacity (y) is preferable itself because it works to draw out the negative electrode performance to the full.
However, if the high-performing positive electrode of the present invention and the conventional negative electrode are simply combined, the positive electrode capacity and the negative electrode capacity become similar to each other. As a result, more hydrogen dissociates from the negative electrode during a charging operation and the domination of the positive electrode in the cell might be destroyed due to a slight deterioration of the negative electrode. Consequently, the improvement of the nickel positive electrode performance does not lead to the improvement of the cell performance.
In the present invention, however, the cell is so constructed that the negative electrode charge depth which is represented by the Equation 2 becomes 80% or lower. With this charge depth, the negative electrode excess capacity (x) is sufficient, so that the hydrogen gas pressure is not strong enough to cause the safety valve to operate. In addition, the deterioration of the negative electrode does not directly lead to the collapse of the domination of the positive electrode. As a result, the deterioration of the cycle characteristic resulting from the operation of the safety valve can be prevented.
As explained hereinbefore, the nickel-metal hydride storage cell of the present invention is composed of a high-performing nickel positive electrode whose non-reactive capacity rate is 16% or lower and a hydrogen-absorbing alloy negative electrode whose charge depth can be properly regulated. This combination of the electrodes can fully draw out the ability of the high-performing nickel positive electrode without causing the occurrence of hydrogen gas dissociation. As a result, a nickel-metal hydride storage cell with a high capacity and an excellent cycle life can be obtained.
Furthermore, the present invention may have another construction as follows:
In the second step, the average valence of the cobalt compound which is contained in the cobalt coating layer is set at greater than divalence. When the cobalt-coated nickel active material contains a cobalt compound whose average valence is greater than divalence, the active material utilization rate can be improved without fail because of its excellent conductivity, and the charging efficiency of the electrodes is improved because the cobalt compound consumes less charging power during a charging operation.
In the third step, the negative electrode non-discharge capacity is set at 40% or lower of the actual cell capacity, and a hydrogen-absorbing alloy which has been surface-treated with an acid aqueous solution is used as the negative electrode active material. In this construction, a decrease in the low-temperature discharge characteristic can be reduced because of the following reason.
As explained above, when the average valence of the cobalt compound is larger than divalence, the cobalt consumes less charging power whereas the cobalt-coated nickel active material becomes more conductive. As a result, the nickel hydroxide non-discharge capacity (c) and the cobalt compound non-discharge capacity (d) of the positive electrode are reduced, and the negative electrode remaining capacity (y) is also reduced. In order to obtain sufficient volume of the actual discharge capacity, it is preferable for the negative electrode remaining capacity to be smaller. However, the hydrogen-absorbing alloy which is used as the negative electrode active material loses its electrochemical reactivity more easily at a low temperature than the nickel active material which is used for the positive electrode. Therefore, when the negative electrode remaining capacity is set at too small a degree, the cell becomes controlled by the negative electrode during a discharging operation at a low temperature. Consequently, sufficient discharging capacity cannot be taken out, that is, the performance of the nickel positive electrode cannot be fully drawn out.
We inventors of the present invention have found through examinations that the negative electrode remaining capacity is about 42% of the actual cell capacity in this type of conventional cell, and that when the negative electrode remaining capacity is 40% or less of the actual cell capacity, the low-temperature discharge characteristic has a problem. Based on the results, we have tried various measures for improving the low-temperature discharge characteristic of a hydrogen-absorbing alloy electrode. As a result, we have found that a hydrogen-absorbing alloy electrode which is filled with a hydrogen-absorbing alloy whose surface has been treated with an acid aqueous solution is suitable for the high-performing nickel electrode which is filled with the cobalt-coated nickel active material. We also have found that the combination of these electrodes makes it possible to maintain an excellent low-temperature discharge characteristic even if the negative electrode remaining capacity (v) is 40% or lower of the actual cell capacity (b).
To be more specific, when a hydrogen-absorbing alloy is subjected to a surface treatment by using an acid aqueous solution whose pH value is within a range of 0.5 to 3.5, its activity and low-temperature discharge performance are enhanced. Consequently, the use of such a hydrogen-absorbing alloy as the negative electrode prevents the low-temperature discharge characteristic from being severely deteriorated even if the negative electrode remaining capacity (y) is 40% or lower of the actual cell capacity (b). In short, a cell can have a high capacity without sacrificing the low-temperature discharge characteristic.
The reason for the enhancement of the low-temperature discharge characteristic through the treatment with an acid aqueous solution is believed to be that the oxide layers which are formed onto the surfaces of the hydrogen-absorbing alloy particles in the pulverizing process is washed out by the acid aqueous solution, and then catalytic-active metal isolation layers (nickel-rich layers) are formed on the surfaces of the alloy particles.
In the second step of the present invention, the alkali aqueous solution has concentrations within a range of 15 to 4 wt %. The alkali aqueous solution having this concentration has a suitable alkali strength and appropriate viscosity, so that it can penetrate through the cobalt-coated nickel active material particles. Consequently, the cobalt compound which is contained the cobalt coating layer can be evenly changed into a cobalt compound whose valence is larger than divalence.
In the alkali heat treatment, the preferable temperatures are within a range of 50 to 150xc2x0 C. At this temperature, the cobalt compound in the cobalt coating layer can change into a high-order cobalt compound of divalent or greater without fail in the presence of oxygen and alkali. It is also possible to improve the crystalline structure of the nickel hydroxide in the cobalt coating layer.